where one user, probably the plugin owner or one related to it rated constantly with 5 stars to keep up the fictional quality of his plugin: Mehedi Hasan I stopped counting how often he was actually rating the one and same plugin!

This is not what rating is meant for and it gives a completely wrong picture to those plugins. I suggest you simply DELETE all doubled ratings or as soon as a doubled rating occurs you simply block another rating. Instead, people would be able to modify their rating instead.